Nested Queries

• A very powerful feature of SQL: a WHERE clause can itself contain an SQL query! (Actually, so can FROM and HAVING clauses.)

• To find sailors who’ve not reserved #103, use NOT IN.

• To understand semantics of nested queries, think of a nested loops evaluation: For each Sailors tuple, check the qualification by computing the subquery.

SELECT S.sname

FROM Sailors S

WHERE S.sid IN (SELECT R.sid

FROM Reserves R

WHERE R.bid=103)

Find names of sailors who’ve reserved boat #103:

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